1. Field of the Invention
The present invention relates to a method for packet transmission and prioritization, and more particularly, to a method of constructing and transmitting packets with MIMO (Multiple-input Multiple-Output) configuration in a wireless communication system.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred as user equipments (UEs).
A radio link control (RLC) layer is responsible for data transfer of radio bearers from an upper layer, a Packet Data Convergence Protocol (PDCP) layer, and employs three RLC entities of TM, UM, and AM RLC entities responsible for functions of transfer modes of Transparent Mode (TM), Unacknowledged Mode (UM) and Acknowledged Mode (AM) respectively. An RLC entity generates RLC protocol data units (PDUs) with RLC service data units (SDUs) and exchanges the RLC PDUs with its peer RLC entity via lower layers.
The RLC entity uses three types of RLC PDUs, which are data RLC PDUs, retransmission RLC PDUs, and control RLC PDUs. The transmission priority is control RLC PDUs>retransmission RLC PDUs>data RLC PDUs. The data RLC PDUs in UM are called UMD PDUs, whereas the data RLC PDUs in AM are called AMD PDUs.
A UM RLC entity, either in the E-UTRAN or in the UE, is configured either as a transmitting UM RLC entity or a receiving UM RLC entity. The transmitting UM RLC entity receives RLC SDUs from upper layer and sends RLC PDUs to its peer receiving UM RLC entity via lower layers. The receiving UM RLC entity delivers RLC SDUs to upper layer and receives RLC PDUs from its peer transmitting UM RLC entity via lower layers.
An AM RLC entity, either in the E-UTRAN or in the UE, consists of a transmitting side and a receiving side and supports segmentation, retransmission, sequence check and other functions. The receiving side of the AM RLC entity delivers RLC SDUs to the upper layers and receives RLC PDUs from its peer AM RLC entity via the lower layer. The transmitting side of the AM RLC entity receives RLC SDUs from upper layers and delivers RLC PDUs to its peer AM RLC entity via the lower layer.
The MAC layer, a lower layer of the RLC layer supports functions of mapping between logical channels and transport channels, multiplexing, de-multiplexing, logical channel prioritization, transport format selection, and so on. The MAC layer of the LTE system does not support packet segmentation. An MAC entity of the MAC layer exchanges RLC PDUs, seen as MAC SDUs, with the RLC layer via logic channels and MAC PDUs with the physical layer via transport channels, such as an uplink shared channel (UL-SCH) or a downlink shared channel (DL-SCH). An MAC SDU is an identity title of an RLC PDU when being sent to the MAC layer.
At a particular transmission time interval (TTI) regarded as a particular transmission opportunity, a transport block (TB) is provided for the MAC entity, and the MAC entity determines a total size of RLC PDU(s) according to the TB size.
When the transmitting side of the AM RLC entity forms AMD PDUs from RLC SDUs, the transmitting side segments and/or concatenates RLC SDUs so that the AMD PDUs fit within the total size of RLC PDU(s) indicated by the MAC layer at the particular transmission opportunity. The transmitting UM RLC entity works in the same way as the transmitting side of the AM RLC entity when forming UMD PDUs from RLC SDUs.
In the LTE system, a MIMO (Multiple-input Multiple-Output) function is employed to increase the end-user data rate and cell capacity. A transceiver with MIMO function employs multiple transmitting and receiving antennas to substantially enhance the air interface data rate performance.
When the MIMO configuration is applied to the physical, MAC and RLC layers, multiple TBs are allowed in a transmission opportunity, causing problems of MAC/RLC packet construction and transmission. In the following, five scenarios are provided to describe the problems in the prior art, and the receiver and the transmitter of each scenario both employ two antennas for MIMO operation, known as 2×2 MIMO.
The first scenario is described as below. When an eNB allocates two TBs with each TB size of 500 bytes to a UE in a particular transmission opportunity and an RLC entity (AM or UM RLC entity) in the UE has data to send, the total size of RLC PDUs is 2×500=1000 bytes and thereby the MAC layer of the UE indicates the RLC entity of a 1000-byte RLC PDU allowance for transmission. Meanwhile, the RLC entity has RLC SDUs with a total size of 1500-byte in the transmission buffer. For simplicity, the example herein does not take MAC sub-headers and MAC control elements into consideration for TB payload. In this situation, the RLC entity constructs a 1000-byte RLC PDU and submits the RLC PDU to the MAC layer for transmission. However, the MAC layer is not able to transmit this RLC PDU since the size of the RLC PDU is larger than the TB size.
The second scenario is described as below. When an eNB allocates two TBs with each TB size of 500 bytes to a UE in a particular transmission opportunity and an AM RLC entity in the UE has data to send, the total size of RLC PDUs is 2×500=1000 bytes and thereby the MAC layer indicates the AM RLC entity of 1000 bytes for transmission. Like the first scenario, the second scenario does not count in the corresponding MAC sub-headers and MAC control elements for TB payload. Meanwhile, the AM RLC entity generates a control RLC PDU with 5 bytes and has a retransmission RLC PDU with 600 bytes and also RLC SDUs with 1000 bytes in transmission buffer. In this situation, the AM RLC entity generates an RLC PDU with 395 bytes and submits the control RLC PDU, the retransmission RLC PDU, and the generated RLC PDU to the MAC layer. However, the MAC layer is not able to transmit the retransmission RLC PDU since the size of the retransmission RLC PDU is larger than the TB size.
The third scenario is described as below. When an eNB allocates two TBs with each TB size of 500 bytes to a UE in a particular transmission opportunity and an AM RLC entity in the UE has data to send, the total size of RLC PDUs is 2×500=1000 bytes and thereby the MAC layer indicates the AM RLC entity of two total size of RLC PDU(s) each with 500 bytes for transmission. Like the first scenario, the third scenario does not take the corresponding MAC sub-headers and MAC control elements into account for TB payload. Meanwhile, the AM RLC entity generates a control RLC PDU with 5 bytes and has a retransmission RLC PDU 500 bytes (2 bytes header+498 bytes data) and RLC SDUs with 1000 bytes in the transmission buffer. According to the prior art, the AM RLC entity possibly follows a first-in, first-out rule and therefore thinks that the retransmission RLC PDU is sent with the control RLC PDU. In this situation, the AM RLC entity segments the retransmission RLC PDU into two RLC PDU segments with 495 bytes (4-byte header+491-byte data) and 11 bytes (7-byte data+4-byte header) and also generates an 489-byte RLC PDU including RLC SDUs. The 495-byte RLC PDU segment and the 5-byte control RLC PDU are submitted to the MAC layer for the first TB, while the 11-byte RLC PDU segment and the 489-byte RLC PDU are submitted for the second TB.
In the third scenario, the RLC entity has to segment the retransmission RLC PDU to fit within the TB size due to the first-in, first-out rule of the prior art.
Before description of the fourth and fifth scenarios, a buffer status report (BSR) in the MAC layer is introduced. The BSR is used for providing a serving eNB with information about the amount of data in the UL buffers of a UE. There are three types of BSRs: regular BSR, periodic BSR, and padding BSR. The regular and periodic BSRs are sent via MAC control elements included in a MAC PDU, where the padding BSR is sent by being included padding bits, if needed, of the MAC PDU. Every BSR can be generated in a short or long BSR.
With fundamental knowledge of the BSR, the fourth scenario is described as below. An eNB allocates two TBs with each TB size of 20 bytes to a UE in a particular transmission opportunity and an AM RLC entity in the UE has a STATUS PDU as well as a control RLC PDU with 20 bytes for transmission. A short BSR is triggered and generated with one byte so one of the TBs includes this short BSR, and furthermore the AM RLC entity uses the TB containing this short BSR to transmit the STATUS PDU. This causes a partial STATUS PDU with 19 bytes because the TB cannot afford the original 20-byte STATUS PDU.
A partial STATUS PDU includes part info of the original STATUS PDU, meanings that some PDU NACK (negative acknowledgment) information of the original STATUS PDU cannot be in the particular transmission opportunity. Thus, the transmitter of a peer RLC entity cannot know that corresponding PDUs have been received unsuccessfully for the particular transmission opportunity. The PDU NACK information has to wait for being reported at the next or later transmission opportunity, causing inefficiency in packet transmission.
The fifth scenario is described as below. In the current spec related to MAC layer, only one BSR is sent when any of BSR events is triggered. Assuming that an eNB allocates two TBs each corresponding to one MAC PDU and a regular BSR event is triggered, the MAC entity includes a regular BSR in either one of the two MAC PDUs corresponding to the TBs. In this situation, even if the MAC PDU without regular BSR has space for a padding BSR, the MAC PDU does not include the padding BSR. As a result, if the MAC PDU with the Regular BSR is lost during transmission, the eNB cannot know the UE buffer status and thereby cannot provide enough resource for the UE transmission.